1. Field of the Invention
The present invention relates to materials and components for attenuating light signals in a fiber optic network.
2. Discussion of the Known Art
Light sources are used in fiber optic systems or networks to produce light signals at desired wavelengths and power (intensity) levels. In some instances, however, the intensity of the light signals may be too high for sensitive optical receivers that are connected in the network physically close to the light sources. Accordingly, attenuators are inserted between the light sources and the receivers in order to maintain the received power levels within tolerable limits. Commercially available attenuator devices typically provide fixed values of attenuation, for example, 3 dB, 9 dB, 15 dB, or more. Precision variable attenuators are also available.
Attenuators in which an air gap of a determined length is defined between confronting end faces of two fibers, are known. In the absence of a light guide inside the gap, a light signal leaves the end face of the fiber on the transmission side, and spreads conically within the gap so that the intensity of the signal which remains to illuminate the end face of the confronting fiber is diminished by a desired amount. Because high reflections are produced at the interfaces between the end faces of the confronting fibers and the intervening air gap, a transparent polymeric disk may be inserted to fill the gap so as to reduce reflections and associated signal losses at the fiber end faces. See W. W. King, et al, Plastic-Gap Attenuation, Proceedings NFOEC (2001), at pages 742–51.
Attenuator devices should present as low a value of reflectance as possible when installed in fiber optic networks. Otherwise, light reflected at a transmission input of the device may reflect back to a laser light source and thus cause undesirable noise in the network. Polymeric disks or elements used in attenuator devices should therefore have a refractive index (R.I.) value that closely matches the R.I. of the cores of the associated fibers for the operating wavelength(s). Fiber cores have R.I.s of, e.g., about 1.44 to 1.46 (with a typical value of about 1.45). As is known in the art, the refractive index of the core of an optical fiber depends on the core's material properties and geometrical profile.
Reflectance produced at an interface between a given attenuating element and the core of a confronting fiber end face, is related to the difference between the refractive index of the attenuating element and that of the fiber core. For a step index optical fiber, this reflectance may be expressed as:Reflectance (in dB)=−10 log10[f(nco−np)2/(nco+np)2+(1−f)(ncl−np)2/(ncl+np)2]wherein:                f is the relative fraction of guided power in the fiber core,        nco is the refractive index of the fiber core,        ncl is the refractive index of the fiber cladding,        np is the refractive index of the attenuating element.        
In general, the reflectance at an interface between materials of different refractive indices is given by:Reflectance (in dB)=−10 log10 [(nco−np)2/(nco+np)2]
For high speed networks, reflectance values lower than −45 dB are desirable with values less than −50 dB being preferred. Thus, the refractive index of any material that forms the attenuating element should be in the range of 1.420 to 1.470, and preferably in the range of 1.435 to 1.455, for light signal wavelengths of around 850 nanometers (nm) to 1620 nm, assuming the element is to be deployed with fibers whose core refractive index is typically about 1.45.
The body of a fixed attenuator is usually comprised of two mating connector parts, and each connector part has an axially aligned ferrule in which an associated fiber is contained so that an end face of the fiber is exposed at a distal end of the ferrule. An attenuating element is supported inside the attenuator body so that opposite sides of the element are aligned with the end faces of the fibers, and the fiber end faces are urged by the associated ferrules into contact with both sides of the element when the connector parts are joined to one another. The attenuating element is thus placed in a state of compression, and it should be able to resist deformation over a range of temperatures likely to be encountered during operation. Attenuators must perform reliably at elevated temperatures, typically up to 75 degrees C. under certain test conditions. At high transmission power levels (e.g., around 20 dBm), surface temperatures on the attenuating element may in fact rise to as much as 90 degrees C.
Attenuating elements made of thermoplastic materials also must resist deformation under load, i.e., “creep”, for long periods of time. Polymeric thermoplastic materials usually do resist both creep and short term deformation, provided the operating temperature is at least 10 to 15 degrees C. below a so-called glass transition temperature (Tg) of the material. Tg is defined as the temperature at which an amorphous or semi-crystalline polymer softens due to the onset of long-range coordinated molecular motion. When producing attenuating elements, it is therefore desirable to specify materials having a Tg greater than about 105 degrees C., and preferably at least 110 degrees C.
Polymeric thermoplastic attenuating elements currently known to be used in fiber optic networks have one or both of the following limitations:
1. The elements exhibit a reflectance that is higher than −40 dB (i.e., the difference in R.I. between the element and the cores of the associated fibers is greater than 0.03); and
2. The elements cannot resist deformation under load at normal service temperatures, or at elevated temperatures that result when high power light signals become incident on the element (i.e., the Tg of the material is too low).
U.S. Pat. No. 5,082,345 (Jan. 21, 1992) describes an attenuating element made from polymethylmethacrylate (PMMA). The material has a refractive index of 1.4900 (n20D) which produces, at best, a reflection of −40 dB. The term (n20D) connotes that the refractive index was measured at 20 degrees C. using a Na-D light source having a wavelength of 589 nm.
U.S. Pat. No. 5,619,610 (Apr. 8, 1997) discloses an optical terminating element made of a copolymer of propylene and 4-methyl-1-pentene. The refractive index of the copolymer is 1.463 (n20D), and it obtains a reflection of −50 dB. But the Tg of the material is only 25 degrees C. Therefore, the copolymer is not suitable for use as an attenuating element at temperatures likely to be encountered during operation. See also U.S. Pat. No. 5,818,992 (Oct. 6, 1998) which discloses an optical terminating element made of PMMA and having a Tg greater than 80 degrees C.
U.S. Pat. No. 5,073,615 (Dec. 17, 1991) relates to a heat resistant methacrylate-maleimide copolymer having a Tg of between 105° C. and 131° C. Fiber optic attenuating elements formed of the copolymer (also known as “Acritherm” (tm)) exhibit a relatively high reflectance of typically around −35 dB, however. As noted above, the high reflectance is a result of a mismatch between the index of refraction of the element and the index of refraction of the fiber cores that contact the element. The index of refraction of the element is, in turn, determined by the inherent electronic configuration of its molecules, i.e., the methylmethacrylate (MMA)-imide copolymers that comprise the element. In other words, the R.I. of the element cannot be changed significantly and permanently without changing the element's chemical formulation. See also, U.S. Pat. No. 5,319,043 (Jun. 7, 1994).
In view of the known state of the art, there is a need for an attenuating element that exhibits a reflection of less than −45 dB, a Tg of at least 105 degrees C., and which has insignificant intrinsic loss, i.e., a relatively high transmissivity at the desired operating wavelengths.